A documentary is a factual production, in other words its based on a real story for example if someone died in a documentary it means that he died in real life.
A good example of a documentary movie is MASK.
It's about a boy called rocky dennis (Eric Stoltz) who suffers from a skull deformity, he is accepted without question by his mothers freewheeling biker friends. His mother tries to give him as normal a life as possible but it doesn't work out for him because all his high school hate him.
People can learn a lot about someone or something in a documentary because its based on reality.
The news is a documentary because it tells the public about reality and how its is in the outside world.